1. Field of the Invention
The present invention relates generally to a method for producing a MOS transistor having an HDD profile and a LDD profile.
2. Description of the Related Art
In order to avoid short-channel effects, MOS transistors with short channel lengths are usually produced in such a way that the source/drain regions have an LDD (lightly doped drain) profile and an HDD (heavily doped drain) profile. The LDD profile has a lower dopant concentration and a smaller depth than the HDD profile. However, the LDD profile reaches closer to the gate electrode and dictates the channel length of the MOS transistor. By contrast, the HDD profile has a lower connection resistance.
In order to produce a MOS transistor with LDD and HDD profile, a gate dielectric and a gate electrode are firstly structured on the surface of a substrate. Using the gate stack, and optionally thin spacers (for example 20 nm) as a mask, the LDD profile is produced by implantation. Next, thick spacers are formed on the sides of the gate stack. Using the gate stack with the thick spacers as a mask, the HDD profile is then produced by implantation (for example T. Ohguro et al., VLSI Techn. Dig. (1996), page 132 or Y. Nakahara et al., VLSI Techn. Dig. (1996) page 174).
The minimum depths which can be achieved for the dopant profiles are limited by the heat treatment steps, which are needed for annealing and activating the implanted dopant.